Support latest cohttp and revert API break.

• Support cohtpp 0.19.1. (#119, @rgrinberg)
• Revert API break for the Sync functor. Now still takes only a Store.S
  as parameter. (#120)

SHA and Inflate abstractions

• Changes to the Search API:
  • Remove find_exn
  • the type t has a new case Tree_root to represent tree roots
  • the type path is now structured (not a list of strings anymore).
    this removes the confusing semantics of empty strings in paths.
• Changes to the Global_graph API:
  • closure takes an optional full argument to work over
    commit objects only
  • All the arguments of pack are labelled and its result type
    has changed.
  • Add keys to return the topological sort of keys in the graph.
• Improve the size of pack files received when fetching (#115)
  • advertise the fact that ocaml-git clients support ofs-delta
    and thin-pack
  • Compute a minimal set of haves to send to the server after
    the discovery phase, where we already know all the server
    references (#114)
• Unpack shallow pack files after a fetch
• Full support for shallow packs (#81)
• During fetch, we now respect the "allow-reachable-sha1-in-want"
  server (non-)capability. A proper error is reported to the client
  if that's not the case.
• Parametrize the codebase over the Inflate implementation. Useful
  to change the inflate algorithm (or to not use any at all, which
  would be what we want for big files and/or for efficient writes)
  and to simplify the port to other backends. Store.S implementations
  now expose their Inflate implementation.
• Parametrize the codebase over the SHA implementation. Useful to change
  the SHA algorihm (the unix backend provides SHA256) or simplify the
  port to other backends (the mirage backend uses a pure OCaml implementation
  extracted from uuidm). Store.S implementations now expose their Digest
  implementation (#68)
• Better sync API (#113)
  • Change the arguments of Sync.fetch
  • Remove Sync.clone
  • Add Sync.populate to be called after a fetch to properly populate
    the local Git repository (similar to what git clone does).
  • Add ogit clone --no-checkout and make ogit clone --bare more similar
    to the same git command.
• Support Github http(s) URLs without .git (#111)
• Add a dot_git optional parameter to Store.create to specify where
  the Git metadata should be stored (default is still <root>/.git). This
  is useful to properly support bare repositories (#110)
• Full support for 32 bit platform (ie. continue the patch started in 1.6.2).
• Rework the Git.Pack and Git.Pack_index API to speed-up random access.
• Add a Git.SHA.Array module to work with contiguous arrays of SHA1.
• Rename Git.SHA.lenght to Git.SHA.hex_length to avoid confusion.
• Expose Git.SHA.of_short_hex to create short hashes. Git.SHA.of_hex now
  only accept 40 characters hexa-decimal strings.
• Improve the output of ogit clone by showing progress when the pack file
  is downloaded and when the delta are resolved.

Support 32bit architectures

• Support 32 bit platform by avoiding creating large strings. This also improve
  the performance of reading and synchronizin large pack files
  (#103, @gregtatcam)

Bug fixes in the smart HTTP protocol

• Fix a bug in ogit pull using the smart HTTP protocol when the HTTP temporary
  buffer could sometimes be overfill.
• Avoid closing twice the same fd in the smart HTTP protocol.
• Avoid the GC to close a fd while we are still using a channel built on top of
  it -- this affects the smart HTTP protocol only.
• Add an opam file for the mirage-git package.

Fix regressions in `Sync.clone`, fix reading hashes in `.git/HEAD`, change API to reset the store and start to improve 32bit support

• Allow some references to contain pointer to other references (#96)
• Improve the support for 32bit architectures (#97)
• Add Reference.pp_head_contents and Reference.equal_head_contents.
• Remove Store.clear and replace it by Memory.clear, Memory.clear_all
  and FS.remove. This let users have a finer control over the memory
  consumption of the program over time (related to #90)
• Rename all pp_hum functions into pp.
• Fix regression in Sync.fetch and add unit-tests (running only in slow mode).
• Fix reading of .git/HEAD when the contents is a commit hash.
• Depends on Stringext for all the extra string function needed.

Fix listing of packed references

• Fix listing of packed references (#98)

Fix serialization of dates, support shallow packs, fix (?) memory leak, etc

• Fix handling of empty paths (#89)
• Fix the serialization of dates in commit objects
• Expose Git.Packed_value.PIC.pretty
• Improve the efficiency of Git_unix.FS.remove
• Support shallow packs (#81)
• Fix an mmap leak introduced in 1.5.* (#90)
• Remove the dependency to OUnit for the tests
• Improve the pretty printers and the output of ogit

filesystem expansion and remote refs fixes

• Fix filesystem expansion when a filen ame becomes a directory name or when
  a directory name becomes a file name (#87)
• Fix the order of entries in the tree objects (#86)
• Fix the compilation of tests (#85)
• Fetch all remote refs on synchronize (#83, by @AltGr)

Improve the pack file API, support conduit 0.8.4, support for short hashes, add mirage sync support

• Compatibility with cohttp.0.18. (#80 by @rgrinberg)
• Simplify the mirage sync API to use conduit 0.8.4 (breaking API changes)
• Change ogit cat-file to behave exactly as git cat-file
  The previous command is renamed to ogit cat (#75 by @codinuum)
• ogit now supports short hashes instead of full SHA1 (#75 by @codinuum)
• Add Git.Pack.Raw.read to read raw pack files (#75 by @codinuum)
• Git.Pack_index.t now uses a cache of entries. This is more efficient
  than the previous representation (#75 by @codinuum)
• Add Git.Pack_index.mem to find an entry in the pack index cache
  (#75 by @codinuum)
• Add Git.Pack_index.find_offset to find an offset in the pack index
  cache (#75 by @codinuum)
• Add Git.Packed_value.to_value to unpack a value stored in a pack file
  (#75 by @codinuum)
• Support synchronisation for MirageOS unikernels (#70)

Fix the smart HTTP protocol

• Fix multi round-trips in the smart HTTP protocol. This fixes
  depth-limited clones (#71) and fetches.
• Create the git.http library for abstracting away bits of the
  smart HTTP protocol.
• Add User-Agent in the headers of the smart HTTP protocol. This
  makes bitbucket.org happy. (#66, patch from @vklquevs)